In the present context, plywood refers to plywood, chipboard or a corresponding multi-layer product formed from at least three veneer layers placed one upon the other or crosswise and glued and pressed against each other to form plywood, chipboard or other equivalent wood based sheets.
In prior art, various adhesives for use in woodboard industry are known. An example of known technology is the use of phenolic resin glues, such as phenol-formaldehyde resin, or urea resin glues as adhesives. Another known technique is to add a foaming agent to an adhesive in order to foam the adhesive composition before its delivery for application in the gluing of woodboard. A preferred known foaming agent used is dried blood. A problem with the use of dried blood are the ethical questions involved. Moreover, when dried blood is used, it may be necessary to add a separate surface tension reducing agent to the adhesive composition in order to achieve the desired properties. A further problem in the use of dried blood is that it is added to the filler or hardener of the adhesive composition in a powdery form, which is why it is difficult to achieve a perfect homogeneity of the composition.
Various other adhesive compositions not containing dried blood as a foaming agent have also been tried in woodboard industry. However, so far, no composition of sufficient quality and efficiency has been found that contains no dried blood and has gluing properties and a price and durability suited for use in the manufacture of wood-based boards.
A further problem in the use of prior-art adhesive compositions is their poor durability, which is why the compositions have to be mixed just before use. In addition, the adhesive concentration in prior-art compositions is conventionally low, and therefore eventual impurities can contaminate the adhesive composition more easily.
The object of the present invention is to overcome the above-mentioned drawbacks concerning an adhesive composition in connection with the manufacture of wood-based boards. A specific object of the invention is to disclose a new, more effective and more ethical adhesive composition.
The adhesive composition of the invention is characterized by what is disclosed in the claims.